Solid state imaging systems have been in use for a number of years in high-tech devices such as medical instruments, satellites and telescopes. More recently, imagers have been employed in a wide array of mainstream applications such as digital cameras, camcorders and scanners. Most of these applications use Charge-Coupled Devices ("CCD") to build the underlying solid state image sensors.
For various reasons, CCD-based image sensors are limited or impractical for use in many consumer applications. First, CCDs require at least two polysilicon layers with a buried-channel implant to-achieve their high performance, meaning that they cannot be fabricated using standard CMOS fabrication processes. Second, the level of integration that can be achieved with CCD-based imagers is low since they can not include the devices necessary to integrate them with other devices in the application. Finally, the circuits used to transfer data out of the image array to other devices on the system board, such as Digital Signal Processors ("DSPs") and other image processing circuits, have a large capacitance and require voltages higher than the other circuits. Since the currents associated with charging and discharging these capacitors are usually significant, a CCD imager is not particularly well suited for portable or battery operated applications.
As such, less expensive image sensors fabricated out of an integrated circuits using standard CMOS processes are desirable. Essentially, with a CMOS type imager sensor, a photo diode, photo transistor or other similar device is employed as a light detecting element. The output of the light detecting element is an analog signal whose magnitude is approximately proportional to the amount of light received by the element. CMOS imagers are preferred in some applications since they use less power, have lower fabrication costs and offer higher system integration compared to imagers made with CCD processes. Moreover, CMOS imagers have the added advantages that they can be manufactured using processes similar to those commonly used to manufacture logic transistors.
An important signal processing circuit is the analog to digital convertor ("ADC"). In the last few years, CMOS imagers have been developed with the ADC on the imager itself. The optimal place for the ADC is immediately after the photosensor, i.e., on the pixel itself. An example of a prior CMOS image sensor is described in the article entitled "A 128 by 128 Pixel CMOS Area Image Sensor With Multiplex Pixel Line A/D Conversion", IEEE 1996 Custom Integrated Circuits Conference, Yang, David X. D., Fowler, Boyd, Gamal, EL Abbas. In their article, the authors describe an image sensor consisting of an array of pixel blocks wherein each block further consists of a group of four nearest neighbor pixels sharing a single Analog to Digital ("A/D") convertor.
A limitation inherent to such sensors is the use of over-sampling A/D conversion methods which require a clock rate well above the image frame rate. The need to keep the conversion rate high in such image sensors requires a substantial amount of drive current making the sensor impractical for many mainstream applications including battery powered or portable devices.
Another problem common to prior art CMOS image sensors is the amount of fixed pattern noise due to beta variations from pixel to pixel which can often be seen with the naked eye. Other undesirable features of prior art CMOS image sensors include large comparator offsets, high complexity of the A/D conversion circuitry, high power dissipation and the inability to achieve a non-linear response for certain applications.